Parallel opening hinges are well known, as illustrated by way of example to European Patent No. 0968349 for a vent, the disclosure of which is herein incorporated by reference in its entirety.
When a heavy window sash, say perhaps between 100-200 kilograms in weight, is to be supported for opening parallel to a fixed frame, a number of parallel opening hinges are typically used. For example, the long sides of the vent may use two or even more separate parallel hinges and the shorter sides of the vent may use one or two separate parallel hinges to support and control the opening of the vent. Although these systems work, it has been noted that the weight of a sash can cause a certain amount of dropping and falling out of the upper part of the sash relatively to the fixed frame. This can result in the vent opening out of parallel and being unsightly, and is certainly not ideal. Also, due to the weight of sashes now being carried by parallel hinges, a certain degree of “crabbing” can occur as a slider moves along a track of a parallel hinge during opening and/or closing of the vent. Once again, this is not ideal and essentially spoils the smooth running of the hinge system whether it is driven by manual operation or an electric motor. It has also been noted that too much free play in a central pivot joint between two links of a parallel opening hinge can also reduce the smooth operation of the hinge system.